A unit of measure used for very small quantities, it is equal to the ratio of the weight or volume of one component of a mixture to a billion weights or volumes of the mixture. When based on weight (ppbw), it is equal to the weight or mass of the...

Unit of atmospheric pressure named in honor of Blaise Pascal (1632-1662), whose experiments greatly increased knowledge of the atmosphere. A pascal is the force of one newton acting on a surface area of one square meter. It is the unit of pressure...

A system sensing only radiation emitted by the object being viewed or reflected by the object from a source other than the system. See active system. Source: NASA (http://earthobservatory.nasa.gov/Glossary)

The point in the orbit of a planet or comet which is nearest the Sun (as opposed to the aphelion, which is the point in the orbit farthest from the Sun). Source: NASA (http://earthobservatory.nasa.gov/Glossary)

A symbol for the degree of acidity or alkalinity of a solution. Expressed as a negative logarithm of the hydrogen ion concentration in a solution, pH = -log10[H+]. If the hydrogen ion concentration of a solution increases, the pH will decrease, and...

Microscopic, plant-like marine organisms (mostly algae and diatoms), which are responsible for most of the photosynthetic activity in the oceans. Phytoplankton are the base of the ocean food web, and they absorb about as much carbon dioxide from the...

The fraction of incident solar radiation that is reflected by a planet and returned to space. The planetary albedo of the Earth-atmosphere system is approximately 30 percent, most of which is due to backscatter from clouds in the atmosphere. Source...

A satellite that can carry instruments. See bus. The same term is applied to automatic weather data transmitters installed on buoys, balloons, ships, and planes, and mounted in remote areas. Source: NASA (http://earthobservatory.nasa.gov/Glossary)